1. Field of the Invention
This invention is in the field of jet aerators employing coaxial nozzle assemblies wherein liquid to be treated is injected through the inner set of nozzles and aspirates substantial quantities of air or other oxidizing gas before being ejected from the module through the outer set of jet nozzles, causing a turbulent interaction between the gas and the liquid streams.
2. Description of the Prior Art
There are a large number of well known liquid treatment processes which are designed to introduce oxygen into a liquid such as a waste liquid for purification purposes, particularly for the reduction of the biochemical oxygen demand (B.O.D.) of the liquid. One of the most widely used of such systems for treating sewage is the activated sludge system in which incoming sewage is subjected to an initial settling in a presettling tank. Raw sludge is withdrawn from the presettling tank and passes to a sludge digestion system. The effluent from the presettling tank goes to an aeration tank where it is treated with the activated sludge and usually passed to a final settling tank where a relatively pure effluent is withdrawn. The residue from the settling tank is then sent to the sludge digestion stage or to the initial aeration tank. The efficient contact of the sewage with oxygen in the aeration tank is vital to the efficient operation of this process.
There are a wide variety of well known methods and systems for introducing oxygen into the liquid to be treated. One of the simplest and least expensive in terms of operating costs involves merely retaining the waste liquid in a pond or tank or the like which is open to the atmosphere. Some of the oxygen from the air will be transferred into the waste liquid and ultimately will reduce its. B.O.D. but this process is extremely slow and is not generally feasible because of its inherently small capacity.
Another method for reducing the B.O.D. of the waste liquid involves the retention of the liquid in a pond or the like and the introduction of air or oxygen under pressure through a tube or diffuser or the like directly into the waste liquid below the surface thereof. While this system increases the rate at which the B.O.D. is reduced, it does involve operating costs in the form of horsepower consumption for the air fans or compressors or the like.
Another method which has been used in the past involves large rotary brushes which are partially submerged in the liquid and which are slowly rotated so that the brushes themselves ultimately rise out of the liquid and into the atmosphere and entrain air, and then descend into the liquid, whereupon a portion of the entrained air is transferred to the waste liquid. This system also involves substantial consumption of power and in addition requires one or more large rotating mechanical brushes which are subject to deterioration and wear of moving parts after prolonged periods of use.
One of the most efficient methods of introducing air into a waste liquid involves the use of jet aerators which employ the venturi principle. In this method, liquid is pumped through a high velocity liquid jet nozzle, thereby generating a reduced pressure on the discharge side of the nozzle. A mixing chamber or zone surrounds the discharge of the liquid nozzle and is in communication with the atmosphere, either directly or indirectly through an air compressor. The high velocity liquid jet which is discharged from the liquid nozzle mixes with or entrains the air, the air in the mixing zone and the air-liquid mixture is then discharged through a liquid-air nozzle directly into the waste liquid below the surface thereof. In terms of oxygen-liquid transfer, this system involving jet aerators produces greater efficiency than the previously mentioned systems. The introduction of air or oxygen into a waste liquid by means of jet aerators is particularly attractive in terms of system capacity, efficiency and operating costs.
One highly efficient jet aerator type of installation is that known as an "eddy mixer" wherein a plurality of jet aerator nozzles is disposed in a tank and the liquid to be treated is passed through a liquid nozzle whereupon it entrains substantial quantities of air and is then discharged through a liquid-gas nozzle, creating turbulent eddy currents which very efficiently mix the air of other oxidizing gas into the liquid. Heretofore, such aerator assemblies have been relatively cumbersome and expensive since they have been made of relatively massive castings or forgings composed of bronze, stainless steel, or other expensive materials.